Conversion of ethane to ethylene

ABSTRACT

Methods of converting ethane to ethylene at relatively low temperatures are described. Ir02-based catalysts are used in the conversion. Methods of converting a base gas to a first gas by exposing the base gas to an IrO2-based catalyst and forming the first gas are described. The base gas can be an alkane. The first gas can include an alkene, an alkyne, an alcohol, an aldehyde, or combinations thereof.

CLAIM OF PRIORITY TO RELATED APPLICATION

This application is a continuation of co-pending U.S. applicationentitled “Conversion of Ethane to Ethylene” having Ser. No. 16/959,325filed on Jun. 30, 2020, which application is the 35 U.S.C. § 371national stage of PCT application having serial numberPCT/US2019/014114, filed on Jan. 18, 2019. The PCT application alsoclaims the benefit of and priority to U.S. Provisional Application Ser.No. 62/747,359, having the title “CONVERSION OF ETHANE TO ETHYLENE”,filed on Oct. 18, 2018, and further claims the benefit of and priorityto U.S. Provisional Application Ser. No. 62/618,813, having the title“CONVERSION OF ETHANE TO ETHYLENE”, filed on Jan. 18, 2020, thedisclosure of each of which is incorporated herein by reference in theirentireties.

GOVERNMENT FUNDING

This invention was made with Government support under DE-FG02-03ER15478awarded by the Department of Energy. The Government has certain rightsin this invention.

BACKGROUND

Developing catalysts that can directly convert ethane to ethylene isgaining increasing interest due to the availability of light alkanesfrom shale gas as well as the increasing demand for ethylene. However,the catalysts that have been investigated to date do not achievesufficient activity and selectivity to be utilized at the industrialscale.

SUMMARY

Embodiments of the present disclosure provide for methods of convertinga base gas to a first gas by exposing the base gas to an IrO₂-basedcatalyst and forming the first gas. The base gas can be an alkane. Thefirst gas can include an alkene, an alkyne, an alcohol, an aldehyde, orcombinations thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the present disclosure can be better understood withreference to the following drawings. The components in the drawings arenot necessarily to scale, with emphasis instead being placed uponclearly illustrating the principles of the disclosure. Moreover, in thedrawings, like reference numerals designate corresponding partsthroughout the several views.

FIGS. 1A-1B provide examples of TPRS spectra of C₂H₆, C₂H₄, CO, CO₂ andH₂O obtained after adsorbing C₂H₆ on IrO₂(110) at 90 K to reach initialC₂H₆ coverages of (FIG. 1A) 0.11 ML and (FIG. 1B) 0.27 ML.

FIG. 2 is an example of TPRS product yields as a function of the initialcoverage of C₂H₆ adsorbed on IrO₂(110) at 90 K, including the initialcoverage of C₂H₆ σ-complexes (desorbed+reacted), the reacted yield ofC₂H₆, the C₂H₄ yield and the yield of ethane that oxidizes (0.5*CO_(x)).

FIG. 3A is an example showing TPRS traces of m/z=27 and 44 obtainedafter adsorbing ˜0.14 ML of C₂H₆ at 90 K on a (nominally) cleanIrO₂(110) surface (blue) and an IrO₂(110) surface with a hydrogenpre-coverage of 0.32 ML (red). The 27 and 44 amu traces are representedby thick vs. thin lines. FIG. 3B shows the total reacted C₂H₆ yield,oxidized C₂H₆ yield (0.5*CO_(x)) and C₂H₄ yield obtained as a functionof the hydrogen pre-coverage during TPRS for a C₂H₆ coverage of ˜0.13ML.

FIGS. 4A-4B show examples of energy pathways for the dehydrogenation ofC₂H₆ adsorbed on IrO₂(110) as computed using DFT-D3 for surfacesinitially containing (FIG. 4A) zero and (FIG. 4B) two HO_(br) groups.The final reaction step compares the energy changes for C₂H₄ desorption(red) vs. dehydrogenation to a C₂H₃(ad) species (black). A comparison ofthe energetics for these pathways with and without D3 can be found inTable 1.

FIG. 5 is a model representation of top and side view of stoichiometricIrO₂(110) structure. The red and blue atoms represent O and Ir atoms,respectively. Rows of Ir_(cus), Ir_(6f), O_(br), O_(3f) along the [001]crystallographic direction are indicated. The unit cell dimensions a andb are parallel to the [001] and [110] directions of the IrO₂ crystal.

FIGS. 6A-6D TPRS are examples of spectra obtained after adsorbing C₂H₆on IrO₂(110) at 90 K to generate coverages of 0.05, 0.11, 0.18, 0.22,0.27, 0.36 and 0.49 ML. TPRS traces are shown for (FIG. 6A) m/z=27,(FIG. 6B) m/z=28, (FIG. 6C) m/z=29 and (FIG. 6D) m/z=44 and the featuresarising from specific compounds are labeled.

FIGS. 7A-7B are examples of TPRS spectra obtained after exposingIrO₂(110) to a) 0.8 L and b) 1.5 L of C₂H₄ at 90 K.

FIGS. 8A-8D are models of the top and side view of ethane adsorbed onIrO₂(110) in (FIG. 8A) 2η¹ (staggered) (FIG. 8B) 2η¹ (eclipsed) (FIG.8C) η² bridge and (FIG. 8D) η¹ top with a PBE (PBE-D3) binding energiesof 53.7 (107.3), 44.3 (98.9), 14.4 (46.9), 39.9 (79.5) kJ/mol,respectively. The 2η¹ configurations shown in FIGS. 8A and 8B have noimaginary frequencies and thus correspond to minima in the potentialenergy surface, whereas the configurations shown in 8C and 8D each haveone imaginary frequency, indicating that these structures occur atsaddle points on the potential energy surface.

DETAILED DESCRIPTION

Before the present disclosure is described in greater detail, it is tobe understood that this disclosure is not limited to particularembodiments described, as such may, of course, vary. It is also to beunderstood that the terminology used herein is for the purpose ofdescribing particular embodiments only, and is not intended to belimiting, since the scope of the present disclosure will be limited onlyby the appended claims.

Where a range of values is provided, it is understood that eachintervening value, to the tenth of the unit of the lower limit (unlessthe context clearly dictates otherwise), between the upper and lowerlimit of that range, and any other stated or intervening value in thatstated range, is encompassed within the disclosure. The upper and lowerlimits of these smaller ranges may independently be included in thesmaller ranges and are also encompassed within the disclosure, subjectto any specifically excluded limit in the stated range. Where the statedrange includes one or both of the limits, ranges excluding either orboth of those included limits are also included in the disclosure.

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this disclosure belongs. Although any methods andmaterials similar or equivalent to those described herein can also beused in the practice or testing of the present disclosure, the preferredmethods and materials are now described.

As will be apparent to those of skill in the art upon reading thisdisclosure, each of the individual embodiments described and illustratedherein has discrete components and features which may be readilyseparated from or combined with the features of any of the other severalembodiments without departing from the scope or spirit of the presentdisclosure. Any recited method can be carried out in the order of eventsrecited or in any other order that is logically possible.

Embodiments of the present disclosure will employ, unless otherwiseindicated, techniques of chemistry, inorganic chemistry, materialscience, and the like, which are within the skill of the art. Suchtechniques are explained fully in the literature.

The following examples are put forth so as to provide those of ordinaryskill in the art with a complete disclosure and description of how toperform the methods and use the compositions and compounds disclosed andclaimed herein. Efforts have been made to ensure accuracy with respectto numbers (e.g., amounts, temperature, etc.), but some errors anddeviations should be accounted for. Unless indicated otherwise, partsare parts by weight, temperature is in ° C., and pressure is inatmosphere. Standard temperature and pressure are defined as 25° C. and1 atmosphere.

Before the embodiments of the present disclosure are described indetail, it is to be understood that, unless otherwise indicated, thepresent disclosure is not limited to particular materials, reagents,reaction materials, manufacturing processes, or the like, as such canvary.

It is also to be understood that the terminology used herein is forpurposes of describing particular embodiments only, and is not intendedto be limiting. It is also possible in the present disclosure that stepscan be executed in different sequence where this is logically possible.

It must be noted that, as used in the specification and the appendedclaims, the singular forms “a,” “an,” and “the” include plural referentsunless the context clearly dictates otherwise. Thus, for example,reference to “a support” includes a plurality of supports. In thisspecification and in the claims that follow, reference will be made to anumber of terms that shall be defined to have the following meaningsunless a contrary intention is apparent. As used herein, “alkane” refersto a saturated aliphatic hydrocarbon which can be straight or branched,having 1 to 40, 1 to 20, 1 to 10, or 1 to 5 carbon atoms, where thestated range of carbon atoms includes each intervening integerindividually, as well as sub-ranges. Examples of alkane include, but arenot limited to methane, ethane, propane, butane, pentane, and the like.Reference to “alkane” includes unsubstituted and substituted forms ofthe hydrocarbon.

As used herein, “alkyne” refers to straight or branched chainhydrocarbon groups having 2 to 40, 2 to 20, 2 to 10, or 2 to 5 carbonatoms and at least one triple carbon to carbon bond. Reference to“alkyne” includes unsubstituted and substituted forms of thehydrocarbon. As used herein, “alkene” refers to an aliphatic hydrocarbonwhich can be straight or branched, containing at least one carbon-carbondouble bond, having 2 to 40, 2 to 20, 2 to 10, or 2 to 5 carbon atoms,where the stated range of carbon atoms includes each intervening integerindividually, as well as sub-ranges. Examples of alkene groups include,but are not limited to, ethene, propene, and the like.

As used herein, “alcohol” refers to a R—OH, where R can be alkyl group.An alkyl group refers to a straight or branched moiety, having 1 to 40,1 to 20, 1 to 10, or 1 to 5 carbon atoms, where the stated range ofcarbon atoms includes each intervening integer individually, as well assub-ranges. Examples of alcohol include, but are not limited tomethanol, ethanol, propanol, butane, pentanol, and the like. Referenceto “alcohol” includes unsubstituted and substituted forms of thealcohol.

As used herein, “aldehyde” refers to a R(O)H, where R can be alkylgroup. An alkyl group refers to a straight or branched moiety, having 1to 40, 1 to 20, 1 to 10, or 1 to 5 carbon atoms, where the stated rangeof carbon atoms includes each intervening integer individually, as wellas sub-ranges. Examples of aldehyde include, but are not limited toformaldehyde, acetaldehyde, propionaldehyde, butyraldehyde, and thelike. Reference to “aldehyde” includes unsubstituted and substitutedforms of the aldehyde.

Discussion

Embodiments of the present disclosure provide for methods of convertingalkanes (e.g., ethane) to a first gas (e.g., an alkene, an alkyne, analcohol, an aldehyde, or a combination thereof), systems, catalysts, andthe like.

Current industrial practice is to use the so-called steam-crackingprocess to produce ethylene from ethane. In the steam-cracking process,the ethane reactant is diluted in steam and heated to high temperature(˜800° C.) to promote ethane pyrolysis. Steam-cracking is highly energyintensive and produces large quantities of CO and CO₂ as byproducts.Equipment costs and maintenance are also substantial. A catalyticprocess that can efficiently and selectively convert ethane to ethyleneat lower temperature would have significant economic and environmentalbenefits.

Developing efficient methods for directly converting ethane to ethylenehas gained increasing interest due to the availability of shale gas andthe increasing demand for ethylene. Ethylene is produced and sold in thelargest quantities among all products generated by the petrochemicalindustry. Realizing the efficient and selective conversion of ethane toethylene is important for improving the utilization of hydrocarbonresources, yet remains a major challenge in catalysis. Catalysts thathave been investigated to date are not sufficiently efficient andselective to be put into industrial practice for converting ethane toethylene. The methods described in the present disclosure provide moreefficient alternatives for generating ethylene from ethane.

Embodiments of the present disclosure provide for methods of convertinga base gas to a first gas. The method includes exposing the base gas toan IrO₂-based catalyst and forming the first gas. The base gas caninclude a C1 to C8, or C1 to C6, or C1 to C5, or C1 to C4 alkane. In anexample, the alkane can be methane, ethane, propane, and combinationsthereof. For example, the base gas may be natural gas. The first gas caninclude a C1 to C8, or C1 to C6, or C1 to C5, or C1 to C4 alkene, a C1to C8, or C1 to C6, or C1 to C5, or C1 to C4 alkyne, a C1 to C8, or C1to C6, or C1 to C5, or C1 to C4 alcohol, a C1 to C8, or C1 to C6, or C1to C5, or C1 to C4 aldehyde, or a combination thereof. In variousembodiments, the base gas can be ethane or methane, while the first gascan include ethylene, methanol, formaldehyde, or combinations thereof.

In some embodiments, the IrO₂-based catalyst can be partiallyhydrogenated (e.g. can be pre-hydrogenated prior to exposure to the basegas). In various embodiments, the IrO₂-based catalyst can be IrO₂(110).

In an aspect, it was found that ethane forms strongly-boundsigma-complexes on the IrO₂(110) surface and that a large fraction ofthe complexes undergo C—H bond cleavage at temperatures below 200 Kduring temperature programmed reaction spectroscopy (TPRS) experiments.It was found that continued heating causes as much as 40% of thedissociated ethane to dehydrogenate and desorb as ethylene near 350 K,with the remainder oxidizing to CO and CO₂. It was also determined thatpartial hydrogenation of the IrO₂(110) surface enhances ethyleneproduction from ethane while suppressing oxidation to CO_(x) species.These experiments reveal that IrO₂(110) exhibits an exceptional abilityto promote ethane dehydrogenation to ethylene just above roomtemperature, and demonstrate that controlled prehydrogenation of theIrO₂(110) surface is an effective approach for increasing selectivitytoward ethane conversion to ethylene rather than CO and CO₂.

Prehydrogenation, as described herein, is attained via the followingprocess. First, H₂ is adsorbed on the IrO₂(110) surface at 90 K and thenheated to 380 K. The H₂ exposure to the surface can be varied to controlthe H surface concentration. Heating to 380 K has two effects: itincreases the quantity of surface OH groups and also causes H atoms tovacate the Ir sites needed for alkane adsorption and activation. The H₂dissociation process may be represented as H₂—Ir+O to H—Ir+OH. Heatingcauses the following: H—Ir+O to Ir+OH. Lastly, the temperature islimited to 380 K to avoid reduction of the surface via 2OH to H₂O(g).The precise adsorption temperature (90 K) is not critical and neither isheating. However, the heating step helps achieve a more desirablesurface in which a large fraction of Ir atoms are vacant and themajority of the surface H-atoms are bound to O-atoms, giving OH groups.

In various embodiments, the base gas (such as an alkane) can be exposedto an IrO₂-based catalyst at a temperature of about 90 to 500 K, about200 to 400 K, or about 350 K. Our work shows that the IrO₂(110) surfaceactivates ethane C—H bonds at temperatures below 200 K, and that a largeportion of the dissociated ethane dehydrogenates and desorbs as ethyleneat temperatures of about 300 to 450 K. Selectivity toward ethyleneproduction during TPRS increases with increasing initial ethanecoverage. No other material is capable of achieving ethane to ethyleneconversion at these low temperatures and with the efficiency realized onIrO₂(110). The discovery of this efficient chemical transformation haspotential to serve as the basis for developing IrO₂-based catalysts thatcan directly and efficiently promote the conversion of ethane toethylene. The successful development of such a process for industrialapplication would have transformative impact on the commercialproduction of ethylene. The economic benefits could be enormous.

Nearly 40% of the adsorbed ethane converts to ethylene during TPRS whenthe ethane layer is initially saturated.

DFT calculations confirm that “bridging” HO groups of the IrO₂(110)surface are effectively inactive as H-atom acceptors, and thatconversion of surface bridging-O atoms to HO groups hinders extensivedehydrogenation of adsorbed ethane-derived species and promotes ethylenedesorption.

Polycrystalline and nanoparticle forms of IrO₂ and IrO₂ surface facetswill promote facile ethane dehydrogenation and conversion to ethylene.This discovery demonstrates that pairs of coordinatively unsaturated(cus) Ir and O atoms at the surface are needed to achieve the observedreactivity on IrO₂(110). Such sites are present on the surface of otherforms of IrO₂.

In some embodiments, the IrO₂-based catalyst can have the formulaIr_(x)M_(y)O_(z), where M is selected from Ru, Ti, Re, Nb, Ta, Os, Pt,Pd, Cu, Ag, Au, Rh, Cr, Mn, Ni, Fe, Co or a combination thereof, andwhere z is between 1 and 2 and x+y≤1. Mixed oxides that include IrO₂moieties will promote facile ethane dehydrogenation and conversion toethylene, including solid oxide solutions in which Ir atoms and a secondmetal cation (M) are present on separate cation lattice sites of theoxide structure in a range of compositions.

In some embodiments, the IrO₂ based catalyst can have the formulaIr_(x)O_(y)X_(z), where X is selected from F, Cl, Br, I, S, Se, Te, or acombination thereof, wherein x≤1 and y+z is between 1 and 2. IrO₂deposited onto another metal oxide (e.g. SiO₂, Al₂O₃, TiO₂, MgO, CaO,CeO₂, zeolites or a combination thereof), referred to as a supportoxide, or various non-oxide support materials (e.g. carbon) will promotefacile ethane dehydrogenation and conversion to ethylene.Anion-substituted, solid mixtures of the form Ir_(x)O_(y)X_(z) willpromote facile ethane dehydrogenation and conversion to ethylene, whereX represents an element that replaces a fraction of the O-atoms in theanion sub-lattice.

Various forms of IrO₂, as listed above, are able to promote theselective dehydrogenation and oxidation of methane to desirable organicproducts, including but not limited to ethylene, methanol andformaldehyde.

Various forms of IrO₂, as listed above, are able to promote theselective dehydrogenation and oxidation of higher alkanes in pure formand mixtures to generate desirable organic products, including but notlimited to alkenes, alkynes, alcohols, aldehydes and other value-addedspecies (e.g., ketones, esters, ethers and organic acids).

Various forms of IrO₂, as listed above, can promote the steady-state,selective dehydrogenation and oxidation of alkanes, including methane,ethane and higher alkanes, to value-added products. Mixtures of thealkane(s) of interest and O₂ can be continuously fed to a reactorcontaining the IrO₂-based catalyst, and continuously produce value-addedproducts.

EXAMPLES

Now having described the embodiments of the disclosure, in general, theexamples describe some additional embodiments. While embodiments of thepresent disclosure are described in connection with the example and thecorresponding text and figures, there is no intent to limit embodimentsof the disclosure to these descriptions. On the contrary, the intent isto cover all alternatives, modifications, and equivalents includedwithin the spirit and scope of embodiments of the present disclosure.

Example 1

Realizing the efficient and selective conversion of ethane to ethyleneis important for improving the utilization of hydrocarbon resources, yetremains a major challenge in catalysis. Herein, ethane dehydrogenationon the IrO₂(110) surface is investigated using temperature programmedreaction spectroscopy (TPRS) and density functional theory (DFT)calculations. The results show that ethane forms strongly-boundσ-complexes on IrO₂(110) and that a large fraction of the complexesundergo C—H bond cleavage during TPRS at temperatures below 200 K.Continued heating causes as much as 40% of the dissociated ethane todehydrogenate and desorb as ethylene near 350 K, with the remainderoxidizing to CO_(x) species. Both TPRS and DFT show that ethylenedesorption is the rate-controlling step in the conversion of ethane toethylene on IrO₂(110) during TPRS. Partial hydrogenation of theIrO₂(110) surface is found to enhance ethylene production from ethanewhile suppressing oxidation to CO_(x) species. DFT predicts thathydrogenation of reactive oxygen atoms of the IrO₂(110) surfaceeffectively deactivates these sites as H-atom acceptors, and causesethylene desorption to become favored over further dehydrogenation andoxidation of ethane-derived species. The study reveals that IrO₂(110)exhibits an exceptional ability to promote ethane dehydrogenation toethylene near room temperature, and provides molecular-level insightsfor understanding how surface properties influence selectivity towardethylene production.

Introduction

Developing catalysts that can directly convert ethane to ethylene isgaining increasing interest due to the availability of light alkanesfrom shale gas as well as the increasing demand for ethylene. Theoxidative dehydrogenation (ODH) of ethane offers advantages overnon-oxidative processes and has been widely studied.¹⁻³ The ODH ofethane occurs in the presence of oxygen and involves the dehydrogenationof ethane to ethylene with concurrent oxidation of the released hydrogento water. The latter step makes the ODH of ethane an exothermic processfor which high conversion is thermodynamically favored at lowtemperature. Furthermore, the presence of oxygen in the reactant streamminimizes catalyst deactivation by coking which can be a significantproblem in non-oxidative routes for ethane dehydrogenation. Variousmetal oxides as well as alkali chlorides are effective in promoting theODH of ethane and propane, with VON-based catalysts generally exhibitingthe most favorable performance.' However, the catalysts that have beeninvestigated to date do not achieve sufficient activity and selectivityto be utilized at the industrial scale.

Initial C—H bond cleavage is widely accepted as the rate-controllingstep in the ODH of ethane, and more generally in the catalyticprocessing of light alkanes.¹ This situation presents a challenge indeveloping catalysts that can selectively dehydrogenate ethane toethylene because the reaction steps that follow initial C—H bondcleavage occur rapidly and can be difficult to control, particularly inthe presence of oxygen. Recently, we have reported that CH₄ undergoeshighly facile C—H bond activation on the IrO₂(110) surface attemperatures as low as 150 K.¹⁰ We find that methane adsorbs as astrongly-bound σ-complex on IrO₂(110) and that C—H bond cleavage occursby a heterolytic pathway wherein the adsorbed complex transfers a H-atomto a lattice oxygen atom, thus affording adsorbed CH₃ and OH groups. Ourresults further show that the resulting methyl groups react with theIrO₂(110) surface via oxidation to CO_(x) and H₂O as well asrecombination with adsorbed hydrogen to regenerate CH₄, with theseproducts desorbing at temperatures above ˜400 K during temperatureprogrammed reaction spectroscopy (TPRS) experiments.¹⁰ Key findings arethat the initial C—H bond cleavage of CH₄ is highly facile and thatsubsequent reaction steps control the overall chemical transformationsof methane on the IrO₂(110) surface. The ability of IrO₂(110) toactivate alkane C—H bonds at low temperature may provide opportunitiesto develop catalysts that are capable of directly and efficientlytransforming light alkanes to value-added products.

In the present example, we investigated the dehydrogenation of ethane onthe IrO₂(110) surface. We find that initial C—H bond cleavage of C₂H₆occurs efficiently on IrO₂(110) at low temperature (˜150 to 200 K) andthat subsequent reaction produces C₂H₄ as well as CO_(x) species duringTPRS, with the C₂H₄ product desorbing between 300 and 450 K. Wedemonstrate that partially hydrogenating the IrO₂(110) surface toconvert a fraction of the surface O-atoms to OH groups enhances theconversion of C₂H₆ to C₂H₄ while suppressing extensive oxidation toCO_(x) species. Our findings show that the controlled deactivation ofsurface O-atoms is an effective means for promoting the selectiveconversion of ethane to ethylene on IrO₂(110) at low temperature.

Experimental Details

Details of the ultrahigh vacuum (UHV) analysis chamber with anisolatable ambient-pressure reaction cell utilized in the present studyhave been reported previously.¹⁰ Briefly, the Ir(100) crystal employedin this study is a circular disk (9 mm×1 mm) that is attached to aliquid-nitrogen-cooled, copper sample holder by 0.015″ W wires that aresecured to the edge of crystal. A type K thermocouple was spot welded tothe backside of the crystal for temperature measurements. Resistiveheating, controlled using a PID controller that varies the output of aprogrammable DC power supply, supports linearly ramping from 80 to 1500K and maintaining the sample temperature. Sample cleaning consisted ofcycles of Ar⁺ sputtering (2000 eV, 15 mA) at 1000 K, followed byannealing at 1500 K for several minutes. The sample was subsequentlyexposed to 5×10⁻⁷ Torr of O₂ at 900 K for several minutes to removesurface carbon, followed by flashing to 1500 K to remove final traces ofoxygen.

We generated an IrO₂(110) film by exposing Ir(100) to 5 Torr of O₂(Airgas, 99.999%) for a duration of 10 minutes (3×10⁹ Langmuir) in theambient-pressure reaction cell at a surface temperature of 765 K. Ourambient-pressure reaction cell is designed to reach elevated gaspressure while maintaining UHV in the analysis chamber.¹⁰ Afterpreparation of the oxide film, we lowered the surface temperature to 600K, and then evacuated O₂ from the reaction cell and transferred thesample back to the UHV analysis chamber. We exposed the film to ˜23 L O₂while cycling the surface temperature between 300 and 650 K to filloxygen vacancies that may be created during sample transfer from thereaction cell to the analysis chamber. This procedure produces ahigh-quality IrO₂(110) surface that has a stoichiometric surfacetermination, contains ˜40 ML of oxygen atoms and is about 3.2 nmthick.¹⁰⁻¹¹

The stoichiometric IrO₂(110) surface consists of parallel rows offivefold coordinated Ir atoms and so-called bridging O atoms, each ofwhich lacks a bonding partner relative to the bulk and is thuscoordinatively unsaturated (cus). Hereafter, we refer to the fivefoldcoordinated Ir atoms as Ir_(cus) atoms and the bridging O-atoms asO_(br) atoms. On the basis of the IrO₂(110) unit cell, the areal densityof Ir_(cus) atoms and O_(br) atoms is equal to 37% of the Ir(100)surface atom density of 1.36×10¹⁵ cm⁻². Since Ir_(cus) atoms are activeadsorption sites, we define 1 ML as equal to the density of Ir_(cus)atoms on the IrO₂(110) surface.

We studied the adsorption of C₂H₆ (Matheson, 99.999%) on clean andhydrogen pre-covered IrO₂(110) using TPRS. We delivered ethane to thesample from a calibrated beam doser at an incident flux of approximately0.0064 ML/s with the sample-to-doser distance set to about 15 mm toensure uniform impingement of ethane across the sample surface. Weprepared hydrogen pre-covered IrO₂(110) by exposing the surface tovarying quantities of H₂ at 90 K, followed by heating to 380 K. We haverecently reported that this procedure enhances the concentration ofHO_(br) groups by promoting the hopping of H-atoms on Ir_(cus) sites toO_(br).¹¹ We estimate that 0.075 to 0.15 ML of H₂ adsorbs from thevacuum background during cooling of the initially clean IrO₂(110)surface, prior to a TPRS experiment. We collected TPRS spectra afterethane exposures by positioning the sample in front of a shielded massspectrometer at a distance of about 5 mm and then heating at a constantrate of 1 K/s until the sample temperature reached 800 K. To ensureconsistency in the composition and structure of the IrO₂(110) layer, thesurface was exposed to 23.3 L of O₂ supplied through a tube doser whilecycling the surface temperature between 300 and 650 K after each TPRSexperiment. Initially, we monitored a wide range of desorbing species toidentify the main products that are generated from reactions of ethaneon IrO₂(110), and found that the only desorbing species are C₂H₆, CO,CO₂, C₂H₄, CH₄ and H₂O. We quantified desorption yields usingestablished procedures as described in the SI.

Computational Details

All plane wave DFT calculations were performed using the projectoraugmented wave pseudopotentials¹² provided in the Vienna ab initiosimulation package (VASP).¹³⁻¹⁴ The Perdew-Burke-Ernzerhof (PBE)exchange-correlation functional¹⁵ was used with a plane wave expansioncutoff of 450 eV. Dispersion interactions are modeled using the DFT-D3method developed by Grimme et al.¹⁶ We find that this method providesaccurate estimates of the adsorption energies of n-alkanes on PdO(101)¹⁷and RuO₂(110)¹⁸ in comparison with TPD-derived values; however, theDFT-D3 calculations overestimate the adsorption energy of methane onIrO₂(110).¹⁰ We find that DFT-D3 calculations using the PBE functionalalso overestimate the binding energies of C₂H₄ and C₂H₆ on IrO₂(110). Wecompare the results of DFT-PBE calculations performed with and withoutdispersion corrections in Table 1, and note that the predictions fromboth methods support the conclusions of this study. We employed fourlayers to model the IrO₂(110) film, resulting in an ˜12 Å thick slabwith an additional 25 Å vacuum to avoid spurious interactions normal tothe surface. The PBE bulk lattice constant of IrO₂ (a=4.54 Å and c=3.19Å) is used to fix the lateral dimensions of the slab. The bottom twolayers are fixed, but all other lattice atoms are allowed to relaxduring the calculations until the forces are less than 0.05 eV/Å. A 2×4unit cell with a corresponding 2×2×1 Monkhorst-Pack k-point mesh isused. In the present study, we define the binding energy, E_(b), of anadsorbed C₂H₆ molecule on the surface using the expression,

E _(b)=(E _(C) ₂ _(H) ₆ +E _(surf))−E _(C) ₂ _(H) ₆ _(/surf)

where E_(C) ₂ _(H) ₆ _(/surf) is the energy of the state containing theadsorbed C₂H₆ molecule, E_(surf) is the energy of the bare surface, andE_(C) ₂ _(H) ₆ is the energy of an isolated C₂H₆ molecule in the gasphase. All reported binding energies are corrected for zero-pointvibrational energy. From the equation above, a large positive value forthe binding energy indicates a high stability of the adsorbed C₂H₆molecule under consideration. We evaluated the barriers for C₂H₆dehydrogenation on the IrO₂(110) surface using the climbing nudgedelastic band (cNEB) method.¹⁹ Our DFT calculations were performed for asingle C₂H₆ molecule adsorbed within the 2×4 surface model of IrO₂(110),and corresponds to an C₂H₆ coverage equal to 12.5% of the total densityof Ir_(cus) atoms and 25% of the Ir_(cus) density within one Ir_(cus)row.

Results and Discussion

TPRS of C₂H₆ adsorbed on IrO₂(110)

Our TPRS results show that the IrO₂(110) surface is highly reactivetoward ethane as more than 90% of the C₂H₆ adsorbed on IrO₂(110)oxidizes to CO, CO₂ and H₂O during TPRS at low initial C₂H₆ coverages(FIG. 1A). The CO₂ and CO products desorb in TPRS peaks centered at 525and 550 K, while H₂O desorbs over a broader feature spanningtemperatures from ˜400 to 750 K. We also observe a small C₂H₆ TPRS peakat 110 K that arises from weakly-bound, molecularly-adsorbed C₂H₆,likely associated with a minority surface phase.

At high initial C₂H₆ coverages, a fraction of the adsorbed C₂H₆dehydrogenates to produce C₂H₄ in addition to undergoing extensiveoxidation to CO and CO₂ (FIG. 1b ). Ethylene desorption accounts forabout 38% of the total amount of C₂H₆ that reacts during TPRS atsaturation of the initial C₂H₆ layer. The C₂H₄ TPRS feature resultingfrom C₂H₆ dehydrogenation on IrO₂(110) exhibits a maximum at 350 K and ashoulder centered at ˜425 K, and most of the C₂H₄ desorbs at lowertemperature than the CO and CO₂ products. Assuming maximum values of thedesorption pre-factors (5.6×10¹⁸, 1.1×10¹⁹ s⁻¹), we estimate that theC₂H₄ peak temperatures of 350 and 425 K correspond to C₂H₄ bindingenergies of 132 and 162 kJ/mol, respectively. Prior studies show thatmaximum desorption pre-factors are appropriate for describing thedesorption of small hydrocarbons from TiO₂(110) and RuO₂(110)surfaces,^(18, 20) where the pre-factors are computed using a modelbased on transition state theory.²¹ We have performed TPRS experimentsfollowing C₂H₄ adsorption on IrO₂(110), and find that C₂H₄ desorbs in abroad feature spanning temperatures from ˜150 to 500 K. The breadth ofthis TPRS feature likely reflects a sensitivity of the C₂H₄ bindingenergy and configuration(s) to the local environment. Because the C₂H₄TPRS feature resulting from C₂H₆ dehydrogenation desorbs over a similartemperature range as C₂H₄ adsorbed on IrO₂(110), we conclude that C₂H₄production from C₂H₆ on IrO₂(110) is a desorption-limited process.

A new C₂H₆ TPRS feature centered at 185 K emerges after the TPRSfeatures generated by the CO, CO₂, C₂H₄, H₂O products first saturate ata total C₂H₆ coverage near 0.20 ML (SI), with this TPRS featuredeveloping two maxima at ˜150 and 175 K as its desorption yield beginsto saturate (FIG. 1B). The C₂H₆ TPRS peak at 110 K grows only slowly asthe total C₂H₆ coverage increases to about 0.35 ML, but a separate peakat 120 K intensifies sharply thereafter (FIG. 6). The C₂H₆ TPRS featureat 150-185 K is consistent with the desorption of relativelystrongly-bound C₂H₆ σ-complexes adsorbed on the Ir_(cus) atoms ofIrO₂(110). Using Redhead analysis with a maximum value of the desorptionpre-factor (5.9×10¹⁷ s⁻¹), we predict a binding energy of 65 kJ/mol forthe C₂H₆ TPRS peak at 185 K. We also estimate a saturation coverage of˜0.30 ML for C₂H₆ σ-complexes on IrO₂(110), based on the amount of C₂H₆that desorbs above ˜135 K plus the total amount that reacts. Ourestimate agrees to within about 20% of the saturation coverage of C₂H₆σ-complexes on RuO₂(110).¹⁸ Because the σ-complexes serve asdissociation precursors (see below), our TPRS results reveal that C₂H₆C—H bond cleavage occurs readily on IrO₂(110) at temperatures between˜150 and 200 K, i.e., in the same range as desorption of the C₂H₆σ-complexes. We are unaware of other materials that exhibit such highactivity toward promoting the C—H bond activation of C₂H₆.

We have recently shown that IrO₂(110) is exceptionally active inpromoting CH₄ C—H bond cleavage at temperatures as low as 150 K.¹⁰ Thepresent results demonstrate a similarly high reactivity of IrO₂(110)toward C₂H₆ activation. Our prior study shows that CH₄ initially adsorbson Ir_(cus) atoms and undergoes C—H bond cleavage by a heterolyticpathway involving H-atom transfer to a neighboring O_(br) atom,producing CH₃—Ir_(cus) and HO_(br) groups. We found that the energybarrier for CH₄ bond cleavage is nearly 10 kJ/mol lower than the bindingenergy of the CH₄ σ-complex, resulting in near unit dissociationprobability for CH₄ on IrO₂(110) at low temperature and coverage. Theresulting CH₃ groups are oxidized by the surface to CO, CO₂ and H₂O thatdesorb in TPRS features that are similar to those observed in thepresent study for C₂H₆ oxidation on IrO₂(110). This similarity suggeststhat common reaction steps control the rates of CO, CO₂ and H₂Oproduction during the oxidation of CH₄ and C₂H₆ on IrO₂(110), afterinitial C—H bond cleavage. We previously reported that CH₄ oxidation toCO, CO₂ and H₂O is favored at low CH₄ coverage, but that recombinativedesorption of CH₄ competes with oxidation at higher initial CH₄coverage.¹⁰ Our current results demonstrate that C₂H₆ alsopreferentially oxidizes during TPRS when the initial C₂H₆ coverage issufficiently low. A key difference is that C₂H₆ dehydrogenates to C₂H₄on IrO₂(110) at high initial C₂H₆ coverage rather than recombinativelydesorbing, and generates C₂H₄ at relatively low temperature (˜300 to 450K).

We show below that the coverage of HO_(br) groups plays a decisive rolein determining the branching between C₂H₆ oxidation and C₂H₄ production.The proposed steps for C₂H₆ activation and subsequent dehydrogenation onIrO₂(110) are the following,

-   Initial C₂H₆ dissociation vs. desorption: C₂H₆(ad)→C₂H₆(g)    C₂H₆(ad)+O_(br)→C₂H₅(ad)+HO_(br)-   C₂H₅ dehydrogenation: C₂H₅(ad)+O_(br)→C₂H₄(ad)+HO_(br)-   C₂H₄ dehydrogenation vs. desorption:    C₂H₄(ad)+O_(br)→C₂H₃(ad)+HO_(br)C₂H₄(ad)→C₂H₄(g)

Ethane initially adsorbs in a molecular state C₂H₆(ad) and forms aσ-complex by datively bonding with Ir_(cus) atoms, and a competitionbetween dissociation and desorption of the C₂H₆(ad) species determinesthe net probability of initial C—H bond cleavage. Our TPRS results showthat dissociation of the C₂H₆(ad) species is strongly favored overdesorption at low C₂H₆ coverages. Since dissociation of the C₂H₆(ad)species requires an O_(br) atom, a decrease in the coverage of O_(br)atoms via conversion to HO_(br) groups may be mainly responsible forC₂H₆ dissociation reaching saturation during TPRS beyond a critical C₂H₆coverage. After initial dissociation, the resulting C₂H₅(ad) species candehydrogenate to C₂H₄(ad) species, and the C₂H₄(ad) species can eitherdesorb or further dehydrogenate via H-atom transfer to an O_(br) atom.Again, the coverage of O_(br) atoms decreases with increasing C₂H₆coverage because an increasing fraction of the O_(br) atoms is convertedto HO_(br) groups via dehydrogenation of the C₂H₆-derived species.According to the proposed reaction steps, C₂H₄ desorption should becomefavored as the O_(br) atom coverage decreases.

Product Yields as a Function of the C₂H₆ Coverage

FIG. 2 shows the initial and reacted TPRS yields of C₂H₆ σ-complexes asa function of the initial C₂H₆ coverage on IrO₂(110) as well as theyields of C₂H₆ that converts to C₂H₄ vs. oxidizing to CO_(x) species. Weset the total reacted yield of C₂H₆ equal to the sum of the C₂H₄ yieldplus one half of the yield of CO+CO₂, where the factor of one halfconverts the CO_(x) yield to the amount of C₂H₆ that oxidizes, and wedefine the initial amount of C₂H₆ σ-complexes as equal to the reactedC₂H₆ yield plus the amount of C₂H₆ that desorbs in the TPRS featureabove ˜135 K. Our results show that 90 to 100% of the strongly-boundC₂H₆ reacts during TPRS as the C₂H₆ coverage increases to ˜0.25 ML, atwhich point the yield of reacted C₂H₆ begins to plateau toward a valueof 0.20 ML and the yield of C₂H₆ σ-complexes that desorb concurrentlyincreases. The reacted C₂H₆ yield corresponds to about 67% of theadsorbed C₂H₆ complexes at saturation. Our results demonstrate that alarge quantity of C₂H₆ reacts on IrO₂(110) during TPRS, and thus supportthe conclusion that initial C—H activation and subsequent reaction occuron the crystalline terraces of IrO₂(110).

Our results further show that C₂H₆ oxidation is strongly favored at lowcoverage, and that C₂H₄ production initiates at moderate coverage as theCO_(x) yield begins to saturate. The yield of oxidized ethane increasesnearly to saturation with increasing C₂H₆ coverage to about 0.15 ML, andthereafter plateaus at a value of about 0.12 ML. Ethylene productionfirst becomes evident at a C₂H₆ coverage above 0.10 ML and increasestoward a plateau value as the total C₂H₆ coverage rises to ˜0.30 ML. Themaximum C₂H₄ yield is equal to 0.08 ML at saturation of the C₂H₆σ-complexes, and represents a large fraction (˜38%) of the C₂H₆ thatreacts on IrO₂(110). The evolution of the product yields with the C₂H₆coverage suggests that the availability of O_(br) atoms plays a decisiverole in determining the reaction pathways that adsorbed C₂H₆ moleculescan access on IrO₂(110). Notably, our current results show that theCO_(x) yield saturates at an O_(br):C₂H₆ ratio close to five; however,the actual minimum O_(br):C₂H₆ ratio needed to promote C₂H₆ oxidation toCO_(x) may be less than five because background H₂ adsorption converts˜0.15 to 0.25 ML of the initial O_(br) atoms to HO_(br) groups prior tothe C₂H₆ TPRS experiment.

Enhanced Selectivity for C₂H₄ Production on H-Covered IrO₂(110)

We find that the selectivity toward C₂H₄ production from C₂H₆ can beenhanced by pre-hydrogenating the IrO₂(110) surface. FIG. 3A comparesTPRS traces of the 27 and 44 amu fragments obtained after adsorbing˜0.14 ML of C₂H₆ on clean IrO₂(110) vs. an IrO₂(110) surface with anestimated H-atom pre-coverage of 0.32 ML. The 27 amu TPRS trace exhibitswell-separated features arising from C₂H₆ and C₂H₄, and the 44 amufeature alone is sufficient for representing the change in CO_(x)production because surface hydrogenation causes similar changes in theCO and CO₂ TPRS features.

Our results show that pre-hydrogenating the surface to a moderate extent(<˜0.4 ML) causes the CO₂ TPRS peak to diminish, while the C₂H₄ TPRSfeature intensifies and skews toward lower temperature, with the maximumshifting from 445 to 350 K. Pre-hydrogenation also causes a C₂H₆ TPRSpeak at ˜175 K to gain intensity, whereas this peak is negligible aftergenerating a moderate C₂H₆ coverage on clean IrO₂(110) (SI). Thesechanges show that pre-hydrogenating IrO₂(110) suppresses C₂H₆ oxidationto CO_(x) species but enhances C₂H₄ production when the H-atompre-coverage is moderate. The concurrent increase in the C₂H₆ TPRS peakat 175 K correlates with the decrease in CO_(x) TPRS yields, and thusdemonstrates that surface pre-hydrogenation causes a fraction of theadsorbed C₂H₆ σ-complexes to desorb rather than oxidize. This behaviorprovides further evidence that adsorbed C₂H₆ σ-complexes serve asprecursors to reaction and that dissociation involves H-atom transfer toO_(br) atoms.

FIG. 3B shows how the total TPRS yield of reacted C₂H₆ as well as theyields of the C₂H₄ and CO_(x) reaction products evolve as a function ofthe initial H-atom coverage on IrO₂(110), for an initial C₂H₆ coverageof 0.13±0.015 ML. We estimate that the nominally clean IrO₂(110) surfacewas covered by ˜0.15 ML of H-atoms prior to ethane adsorption. Ourresults show that the total yield of reacted C₂H₆ decreasesmonotonically with increasing H-atom pre-coverage, indicating thatinitially converting O_(br) atoms to HO_(br) groups suppresses C₂H₆activation on IrO₂(110). The CO_(x) yield decreases sharply andcontinuously from a value of 0.11 to 0.01 ML as the H-atom coverageincreases to about 1 ML. In contrast, however, the C₂H₄ yield increasesfrom ˜0.03 to 0.045 ML with increasing H-atom coverage to ˜0.32 ML andthereafter decreases, reaching a final value of 0.005 ML at saturationof the initial H-atom layer. These changes represent a nearly threefoldincrease in the selectivity for C₂H₄ production, as measured by theratio of ethane that converts to ethylene vs CO_(x) species. The C₂H₄yield begins to fall below its value on the (nominally) clean IrO₂(110)surface when the initial H-atom coverage starts to exceed 0.5 ML. Theevolution of product yields with increasing H-coverage demonstrates thatO_(br) atoms are needed to promote the initial C—H activation of C₂H₆ onIrO₂(110) as well as further dehydrogenation and that the controlleddeactivation of O_(br) atoms by hydrogenation provides a means toenhance reaction selectivity to favor the conversion of ethane toethylene.

Pathways for C₂H₆ Dehydrogenation on IrO₂(110)

We examined several possible C₂H₆ adsorption configurations (FIGS.8A-8D) and predict that C₂H₆ forms a strongly-bound σ-complex onIrO₂(110) by adopting a flat-lying geometry along the Ir_(cus) row inwhich each CH₃ group forms a H-Ir_(cus) dative bond (a 2η¹configuration) and the C₂H₆ molecule effectively occupies two Ir_(cus)sites. This staggered 2η¹ configuration is similar to that predicted byPham et al. but they report an eclipsed C₂H₆ configuration,²² which wefind to be less stable than the staggered configuration by ˜9 kJ/mol(FIGS. 8A-8D). We have previously reported that C₂H₆ complexes onPdO(101) and RuO₂(110) also preferentially adopt the 2η¹configuration.^(18, 23-24)

FIG. 4A shows the energy diagram computed using DFT-D3 for thesequential dehydrogenation of C₂H₆ to C₂H₄ on IrO₂(110), followed byeither C₂H₄ desorption (red) or C₂H₄ dehydrogenation to adsorbed C₂H₃.DFT-D3 predicts that the 2η¹ C₂H₆ complex achieves a binding energy of107 kJ/mol on clean IrO₂(110) and that the barrier for C—H bond cleavagevia H-transfer to an O_(br) atom is only 38 kJ/mol. According to thecalculations C₂H₆ dehydrogenation to produce C₂H₅—Ir_(cus) and HO_(br)species is exothermic by about 97 kJ/mol, and the barrier for reactionis significantly lower than the binding energy of the adsorbed C₂H₆complex (38 vs. 107 kJ/mol). We find that DFT-PBE calculations withoutdispersion corrections underestimate the C₂H₆ binding energy onIrO₂(110), but still predict that the C₂H₆ dissociation barrier is lowerthan the desorption barrier (Table 1). Our calculations thus predictthat C₂H₆ C—H bond cleavage is strongly favored over moleculardesorption on clean IrO₂(110) such that all adsorbed C₂H₆ molecules willdissociate at low temperature, provided that O_(br) atoms are availablefor reaction. This prediction agrees well with our experimental findingthat C₂H₆ dissociates on IrO₂(110) with near unit probability at lowC₂H₆ coverages (FIG. 2).

We find that the adsorbed C₂H₅ group on IrO₂(110) can also dehydrogenateby a low energy pathway wherein the CH₃ group transfers a H-atom to anO_(br) atom, resulting in an adsorbed C₂H₄ species and a HO_(br) grouplocated in the opposing row from the initial HO_(br) group (FIG. 4A).DFT-D3 predicts an energy barrier of 52 kJ/mol for this reaction and anexothermicity of 75 kJ/mol. The barrier for C₂H₅ dehydrogenation isrelatively low because the CH₃ group maintains a H-Ir. dativeinteraction that weakens one of the C—H bonds. The C₂H₄ product adopts abidentate geometry in which a C—Ir_(cus) σ-bond forms at each CH₂ group(i.e., di-σ configuration). Our calculations predict that the C₂H₄species needs to overcome a barrier of 189 kJ/mol to desorb vs. abarrier of 68 kJ/mol to dehydrogenate via H-transfer to an O_(br) atom,affording an adsorbed C₂H₃ species and a third HO_(br) group. Thecalculations thus predict that C₂H₄ dehydrogenation is strongly favoredover C₂H₄ desorption when O_(br) atoms are available to serve as H-atomacceptors. This prediction is consistent with our experimentalobservation that C₂H₆ oxidation occurs preferentially over C₂H₄production at low initial C₂H₆ and HO_(br) coverages.

FIG. 4B shows the computed pathway for C₂H₆ dehydrogenation on IrO₂(110)when two of the four accessible O_(br) atoms are initially hydrogenatedto HO_(br) groups. For these calculations, we hydrogenated O_(br) atomslocated in opposing rows, with each next to a different CH₃ group of theC₂H₆ complex (FIG. 4B). Our calculations predict that hydrogenation ofthe two O_(br) atoms destabilizes the C₂H₆ σ-complex on IrO₂(110) byabout 22 kJ/mol. We have recently reported that the hydrogenation ofO_(br) atoms also destabilizes H₂ complexes on IrO₂(110).¹¹ Ourcalculations also predict that the energy barriers are nearly the samefor C₂H₆ and C₂H₅ dehydrogenation on the initially clean IrO₂(110) vs.pre-hydrogenated IrO₂(110)-2HO_(br) surfaces when reaction occurs byH-transfer to an O_(br) atom (FIGS. 4A, 4B).

Sequential dehydrogenation of C₂H₆ to C₂H₄ on the initialIrO₂(110)-2HO_(br) surface converts all four of the accessible O_(br)atoms to HO_(br) groups, and causes C₂H₄ desorption to become favoredover further dehydrogenation because HO_(br) groups are much lessreactive than O_(br) atoms. The energy barrier for C₂H₄ dehydrogenationvia H-transfer to a HO_(br) group is 152 kJ/mol, compared with 68 kJ/molfor C₂H₄ dehydrogenation to an O_(br) atom. In addition, the reversereaction features an energy barrier of only 5 kJ/mol so the H₂O_(br)species would rapidly transfer a H-atom to C₂H₃ to regenerate theadsorbed C₂H₄ and HO_(br) species. Our DFT calculations thus indicatethat C₂H₄ desorption is favored over dehydrogenation when all of theaccessible O_(br) atoms are hydrogenated to HO_(br). This predictionagrees well with our experimental findings that pre-hydrogenation ofIrO₂(110) promotes the conversion of C₂H₆ to C₂H₄ while suppressing C₂H₆oxidation, and that C₂H₄ production begins to occur on initially cleanIrO₂(110) only at moderate initial C₂H₆ coverages.

Structure of the s-IrO₂(110) Layer on Ir(100)

Bulk crystalline IrO₂ has a tetragonal unit cell with Ir atomssurrounded by an octahedral arrangement of six oxygen atoms and eachoxygen atom is coordinated with three Ir atoms resulting in a trigonalplane. FIG. 5 shows a top and side view of thestoichiometrically-terminated IrO₂(110) surface. The IrO₂(110) surfaceunit cell is rectangular with dimensions of a=3.16 Å and b=6.36 Å, wherea and b are parallel to the [001] and [110] directions of the IrO₂crystal, respectively. The unit cell dimensions may also be expressed asa a=1.16x and b=2.34x, where x=2.72 A is the lattice constant ofIr(100). The IrO₂(110) surface consists of alternating rows of O_(br)and Ir_(cus) that align along the [001] direction. Each of these surfacespecies has one dangling bond due to a decrease in coordination incomparison to bulk IrO₂.

Measurement of Product Yields

We estimate atomic oxygen coverages by scaling integrated O₂ TPD spectrawith those obtained from a saturated (2×1)-O layer containing 0.50 ML ofO-atoms and prepared by exposing the Ir(100)−(5×1) surface to O₂ inUHV.²⁷ To estimate hydrogen coverages, we scaled integrated hydrogendesorption spectra by an integrated TPD spectrum collected from asaturated Ir(100)−(5×1)-H layer containing 0.80 ML of atomic hydrogenthat we prepared by adsorbing hydrogen to saturation on theIr(100)−(5×1) surface at 300 K.²⁸ We performed TPRS experiments of COoxidation on saturated O-covered Ir(100) to estimate the CO₂ desorptionyields. Specifically, we collected O₂ and CO₂ TPRS spectra afterexposing a (2×1)-O layer to a sub-saturation dose of CO and assumingthat the CO₂ yield is equal to the difference between the initial (0.50ML) and final coverages of oxygen as determined from the O₂ TPRS yield.To estimate CO desorption yields, we scaled integrated CO desorptionspectra by an integrated TPD spectrum collected from a saturated c(2×2)layer containing 0.50 ML of CO that we prepare by adsorbing CO tosaturation on Ir(100)−(1×1) at 300 K.^(27,29,30)

We performed TPRS experiments of hydrogen oxidation on partiallyO-covered Ir(100) to estimate the water desorption yields. In theseexperiments, we first collected O₂ and CO₂ TPRS spectra after exposing a(2×1)-O layer to a sub-saturation dose of CO and assuming that theoxygen remaining on Ir(100) is equal to the difference between theinitial oxygen coverage in the (2×1)-O layer (0.50 ML) and the CO₂ yielddetermined from the CO₂ TPRS spectrum. We then collected O₂ and H₂O TPRSspectra after exposing the partially O-covered Ir(100) surface generatedfrom the first step to a saturation dose of hydrogen and assuming thatthe water yield is equal to the difference between the initial andfinial coverage of oxygen determined from the O₂ TPRS yield. We repeatthese calibration TPRS experiments to ensure accuracy in our estimatesof desorption yields. We estimate C₂H₆ and C₂H₄ coverages by scaling theintensity-to-coverage conversion factors determined for CO with relativesensitivity factors reported for the mass spectrometric detection ofthese gases.

TPRS Spectra as a Function of the C₂H₆ Coverage on IrO₂(110)

FIGS. 6A-6D show TPRS spectra of mass fragments m/z=27, 28, 29 and 44obtained as a function of the initial C₂H₆ coverage generated onIrO₂(110) at 90 K. The mass-fragment TPRS spectra clearly illustrate theTPRS features that arise from C₂H₆, C₂H₄ and CO because these speciesdesorb in well-separated temperature ranges. We deconvoluted selectedmass-fragment TPRS spectra to generate the TPRS spectra for C₂H₆, C₂H₄and CO that we report in FIG. 1. This deconvolution involves firstsubtracting the m/z=29 spectrum from the m/z=27 spectrum after rescalingthe m/z=29 spectrum so that the intensities of the TPRS peaks below 250K are equal in the m/z=29 and 27 spectra. This step removes the C₂H₆contribution from the m/z=27 spectrum and generates a TPRS spectrum foronly C₂H₄. We obtain a CO TPRS spectrum by first subtracting the C₂H₆contribution from the m/z=28 spectrum, and then subtracting the C₂H₄contribution as obtained from the corrected m/z=27 spectrum.

As discussed in the example above, the TPRS spectra demonstrate that COand CO₂ production dominates at low C₂H₆ coverage and that thecorresponding CO and CO₂ TPRS peaks are nearly saturated once theinitial C₂H₆ coverage increases to ˜0.15 ML. Ethylene desorption becomesevident at an initial C₂H₆ coverage of ˜0.1 ML and the C₂H₄ TPRS featureintensifies with increasing coverage thereafter until saturating at aninitial C₂H₆ coverage of ˜0.30 ML. We attribute the C₂H₆ TPRS feature at150-185 K to strongly-bound C₂H₆ σ-complexes and estimate that thisfeature saturates when the total C₂H₆ coverage reaches 0.30 ML. The C₂H₆TPRS peak at ˜110 K intensifies slowly as the initial C₂H₆ coverageincreases to ˜0.30 ML and saturates at a yield of only about 0.025 ML,consistent with a minority species. A separate C₂H₆ TPRS peak at ˜120 Kintensifies sharply with increasing C₂H₆ coverage above 0.30 ML, and isconsistent with C₂H₆ adsorbed on O_(br) sites of IrO₂(110) based onsimilar behavior observed during C₂H₆ adsorption on RuO₂(110) andTiO₂(110).^(31,20)

TPRS Spectra from C₂H₄ on IrO₂(110)

FIGS. 7A and 7B show TPRS spectra obtained after exposing IrO₂(110) to0.8 and 1.5 L of C₂H₄ at 90 K, where the 1.5 L exposure causessaturation of the desorption features above 150 K. A fraction of theadsorbed C₂H₄ oxidizes and produces CO, CO₂ and H₂O that desorb in TPRSfeatures above 400 K, where these features are nearly identical to thoseobserved during our TPRS experiments with C₂H₆ as described in thepresent disclosure. The sharp C₂H₄ TPRS peak at 111 K arises fromweakly-bound C₂H₄ molecules that are likely associated with the O_(br)atoms of IrO₂(110) or a minority surface phase. We attribute the broadC₂H₄ TPRS feature between ˜150 and 500 K to C₂H₄ adsorbed strongly onthe Ir_(cus) atoms. At the lower coverage, a distinct C₂H₄ TPRS peak isevident at 450 K. This feature appears as a shoulder at higher coverageand the C₂H₄ desorption rate remains nearly constant between 200 and 400K. Ethylene adsorption on RuO₂(110) also produces a broad C₂H₄ TPRSfeature.³² We suggest that the significant breadth of the C₂H₄ TPRSfeature reflects a sensitivity of the C₂H₄ binding energy to the localenvironment, including the coverage of C₂H₄ molecules, HO_(br) groupsand C₂H₄-derived species that serve as intermediates to CO_(x)formation.

Configurations of C₂H₆ Adsorbed on IrO₂(110) as Predicted with DFT

FIGS. 8A-8D show the four configurations for ethane adsorbed onIrO₂(110) that we identified with DFT. Pham and co-workers report thatthe eclipsed configuration shown in FIG. 8B is the most stableconfiguration of C₂H₆ on IrO₂(110),²² but we find that the staggeredethane configuration with interactions of the two CH₃ groups with O_(br)atoms in opposite rows is more favorable by 9.3 (8.4) kJ/mol with PBE(PBE-D3).

Comparison of DFT-PBE Results with and without Dispersion-Corrections

TABLE 1 Energies (E) and energy barriers (E_(f)) for the adsorption andsequential dehydrogenation of C₂H₆ on IrO₂(110) computed using DFT-PBEwith (DFT-D3) and without (DFT) dispersion-corrections. Each energy Evalue is referenced to the energy an isolated C₂H₆ molecule plus theclean IrO₂(110) surface, i.e., C₂H₆(g) + IrO₂(110). Transition statesare marked with an asterisk in column 1. We note that the state writtenas (C₂H₄(g) + 2O_(br)/2HO_(br))* corresponds to an isolated C₂H₄molecule and the IrO₂(110)—2HO_(br) surface, and is equivalent to thetransition state for C₂H₄ desorption after C₂H₆ dehydrogenation. Thefinal column shows the difference between the energy of each structurecomputed with DFT-D3 and DFT calculations, each using the PBEfunctional. DFT-D3 DFT (PBE) (PBE) DFT-D3 − DFT E E_(f) E E_(f) ΔEStructure (kJ/mol) (kJ/mol) (kJ/mol) (kJ/mol) (kJ/mol) C₂H₆ + 4O_(br)−107.3 −53.7 −53.6 (C₂H₆ + 4O_(br))* −69.4 37.9 −10.5 43.2 −58.9 C₂H₅ +3O_(br)/HO_(br) −204.5 −149.4 −55.1 (C₂H₅ + 3O_(br)/HO_(br))* −152.252.3 −93.9 55.5 −58.3 C₂H₄ + 2O_(br)/2HO_(br) −279.6 −224.1 −55.5(C₂H₄(g) + −90.9 188.7 −81.6 142.5 −9.3 2O_(br)/2HO_(br))* (C₂H₄ +2O_(br)/2HO_(br))* −211.7 67.9 −152.8 71.3 −58.9 C₂H₃ + O_(br)/3HO_(br)−298.3 −238.7 −59.6

Table 1 shows energies and energy barriers computed for the adsorptionand sequential dehydrogenation of C₂H₆ on IrO₂(110) using DFT-PBE with(DFT-D3) and without (DFT) dispersion corrections. The energiesdetermined using DFT-D3 are greater than those computed using DFT by asimilar amount of 57.1±2.3 kJ/mol for each structure that includes anadsorbed hydrocarbon species derived from C₂H₆. The DFT-D3 calculationsusing the PBE functional overestimate the binding energies of C₂H₆ andC₂H₄ on IrO₂(110) determined from our TPRS data, to an extent that issimilar to our previous results for CH₄ and H₂ adsorbed onIrO₂(110).^(10, 11) Calculations using other dispersion-correctedfunctionals also overestimate the binding energy of C₂H₆ on IrO₂(110).²²From TPD, we estimate a binding energy of about 65 kJ/mol for the C₂H₆σ-complex on IrO₂(110), whereas DFT-D3 predicts a value of 107 kJ/molfor C₂H₆ adsorbed on clean IrO₂(110). Similarly, the C₂H₄ TPRS featurefrom 300 to 450 K suggests a C₂H₄ binding energy on IrO₂(110) betweenabout 130 and 165 kJ/mol, while DFT-D3 predicts of a binding energy of189 kJ/mol. Our DFT calculations without dispersion slightlyunderestimate the C₂H₆ binding energy on IrO₂(110) (54 vs. 65 kJ/mol),and the computed C₂H₄ binding energy falls within the wide rangeestimated experimentally. Notably, our DFT-PBE calculations withoutdispersion predict facile C₂H₆ C—H bond cleavage on IrO₂(110) andsupport the conclusion that C₂H₆ conversion to C₂H₄ is favored overoxidation on partially-hydrogenated IrO₂(110). Although our DFTcalculations support the main conclusions of this study, the overbindingpredicted by current dispersion-corrected DFT methods signals a need forfurther development of exchange-correlation functionals that canaccurately predict molecular binding on IrO₂(110).

Discussion

Our results show that C₂H₆ activation is highly facile on IrO₂(110) attemperatures below 200 K, and that further dehydrogenation produces C₂H₄that desorbs between 300 and 450 K. Based on comparison with referenceTPRS data (SI), we conclude that C₂H₄ desorption is the rate-limitingstep in the conversion of C₂H₆ to C₂H₄ on IrO₂(110) during TPRS. Our DFTcalculations support these conclusions as they predict that the barrierfor C₂H₆ C—H bond cleavage on clean IrO₂(110) is lower than that forC₂H₄ desorption by at least 100 kJ/mol. Indeed, we find that theIrO₂(110) surface is exceptionally active in promoting alkane C—H bondcleavage—we estimate a barrier between 35 and 40 kJ/mol for ethaneactivation on IrO₂(110), and our prior work reveals an even lowerbarrier of 28 kJ/mol for CH₄ activation on IrO₂(110).¹⁰ In fact, our DFTresults predict that initial C—H bond cleavage has the lowest barrieramong the reaction steps involved in C₂H₆ conversion to C₂H₄ onIrO₂(110).

In contrast to IrO₂(110), initial C—H bond activation is therate-determining step in the ODH of alkanes on most other oxides.Supported vanadium-oxide based catalysts have been widely studied due totheir favorable performance in promoting the ODH of ethane andpropane.^(1, 9) While the specific values can depend on multiplefactors, barriers for ethane C—H bond cleavage on VO_(x)-based catalystslie in a range from about 120 to 150 kJ/mol,^(5, 25-26) and reactors areoperated at temperatures between 700 and 900 K to achieve optimal ratesand selectivity of alkene production from ethane and propane.² Accordingto DFT, ethylene desorption is the rate-determining step for theconversion of C₂H₆ to C₂H₄ on IrO₂(110) under TPRS conditions becausethe dehydrogenation of adsorbed C₂H₆ and C₂H₅ groups are both facileprocesses on clean IrO₂(110) and the C₂H₄ product binds strongly. Fromour TPRS data, we estimate that the barrier for C₂H₄ desorption fromIrO₂(110) lies between about 130 and 165 kJ/mol, and is thus close tothe values reported for C₂H₆ C—H activation barriers on VON-basedcatalysts. However, since the entropy of activation is much larger forC₂H₄ desorption compared with ethane activation, C₂H₄ desorbs fromIrO₂(110) at lower temperature relative to the temperatures at whichVON-based catalysts would achieve comparable rates of ethane conversionto ethylene.

Our TPRS results show that the desorption of ethylene from IrO₂(110)occurs at lower temperature during TPRS than the reaction-limiteddesorption of H₂O and CO_(x) species resulting from ethane oxidation(FIGS. 1A-1B). A possible implication is that low temperature operationcan enable IrO₂ catalysts to promote the conversion of ethane toethylene at high rates while minimizing CO_(x) production. However, thehigher desorption temperature of H₂O compared with C₂H₄ suggests thatH₂O desorption could be a rate-controlling step in the IrO₂-promotedconversion of C₂H₆ to C₂H₄ under steady-state conditions. While furtherstudy is needed, our results suggest possibilities for achievingefficient and selective conversion of ethane to ethylene at lowtemperature using IrO₂-based catalysts.

Our results also demonstrate that partial hydrogenation of the IrO₂(110)surface enhances ethane conversion to ethylene while suppressingextensive oxidation to CO_(x) species. We find that HO_(br) groups aresignificantly less active than O_(br) atoms as H-atom acceptors, and, asa result, hydrogenating a fraction of the O_(br) atoms limits the extentto which adsorbed hydrocarbons can dehydrogenate and causes C₂H₄desorption to become favored over further dehydrogenation and extensiveoxidation. This behavior provides a viable explanation of the evolutionof TPRS product yields with increasing C₂H₆ coverage. At low C₂H₆coverage enough O_(br) atoms are available to allow each C₂H₆ moleculeto extensively dehydrogenate, and produce intermediates that oxidize toCO_(x) species with further heating. With increasing C₂H₆ coverage, theextent to which C₂H₆ molecules dehydrogenate becomes limited because alarger fraction of O_(br) atoms convert to HO_(br) groups anddeactivate. Consistent with this interpretation, our experimentsdemonstrate that the selectivity toward ethane conversion to ethylenecan be enhanced by partially hydrogenating the IrO₂(110) surface priorto adsorbing ethane. This finding may have broad implications fordeveloping methods by which to modify the selectivity of IrO₂ catalysts.In particular, our results demonstrate that controllably deactivating afraction of the reactive O-atoms of IrO₂ is an effective approach forpromoting the partial dehydrogenation of ethane over extensiveoxidation.

SUMMARY

We investigated the dehydrogenation of ethane on the stoichiometricIrO₂(110) surface using TPRS and DFT calculations. Our results show thatethane forms strongly-bound σ-complexes on IrO₂(110) and that a largefraction of the adsorbed complexes undergo C—H bond cleavage below 200 Kduring TPRS. Our DFT calculations predict that ethane σ-complexes onIrO₂(110) dissociate by a heterolytic mechanism involving H-atomtransfer to a neighboring O_(br) atom, and that the barrier for C—H bondcleavage is lower than the binding energy of the C₂H₆ σ-complex. We findthat the resulting ethyl groups react with the IrO₂(110) surface viaoxidation to CO_(x) species and H₂O as well as dehydrogenation to C₂H₄,with the C₂H₄ product desorbing between 300 and 450 K. Both DFTcalculations and TPRS experiments show that C₂H₄ desorption is therate-limiting step in the conversion of C₂H₆ to C₂H₄ on IrO₂(110) duringTPRS. Our experimental results demonstrate that partially hydrogenatingthe IrO₂(110) surface enhances the conversion of ethane to ethylenewhile suppressing ethane oxidation to CO_(x) species. According to DFT,converting a fraction of the O_(br) atoms to HO_(br) groups causes C₂H₄desorption to become favored over further dehydrogenation becauseHO_(br) groups are poor H-atom acceptors compared to O_(br) atoms. Ourfindings reveal that the IrO₂(110) surface exhibits an unusual abilityto promote the dehydrogenation of ethane to ethylene near roomtemperature during TPRS, and demonstrate that controlled deactivation ofO_(br) atoms is an effective way to promote ethylene production fromethane on IrO₂(110).

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Ratios, concentrations, amounts, and other numerical data may beexpressed in a range format. It is to be understood that such a rangeformat is used for convenience and brevity, and should be interpreted ina flexible manner to include not only the numerical values explicitlyrecited as the limits of the range, but also to include all theindividual numerical values or sub-ranges encompassed within that rangeas if each numerical value and sub-range is explicitly recited. Toillustrate, a concentration range of “about 0.1% to about 5%” should beinterpreted to include not only the explicitly recited concentration ofabout 0.1% to about 5%, but also include individual concentrations(e.g., 1%, 2%, 3%, and 4%) and the sub-ranges (e.g., 0.5%, 1.1%, 2.2%,3.3%, and 4.4%) within the indicated range. In an embodiment, the term“about” can include traditional rounding according to significant figureof the numerical value. In addition, the phrase “about ‘x’ to ‘y’”includes “about ‘x’ to about ‘y’”.

Unless defined otherwise, all technical and scientific terms used havethe same meaning as commonly understood by one of ordinary skill in theart to which this disclosure belongs. Although any methods and materialssimilar or equivalent to those described can also be used in thepractice or testing of the present disclosure, the preferred methods andmaterials are now described.

Embodiments of the present disclosure will employ, unless otherwiseindicated, techniques of separating, testing, and constructingmaterials, which are within the skill of the art. Such techniques areexplained fully in the literature.

It should be emphasized that the above-described embodiments are merelyexamples of possible implementations. Many variations and modificationsmay be made to the above-described embodiments without departing fromthe principles of the present disclosure. All such modifications andvariations are intended to be included herein within the scope of thisdisclosure and protected by the following claims.

1. A method of converting a base gas to a first gas, comprising:exposing the base gas to an IrO₂-based catalyst, and forming the firstgas, wherein the base gas is an alkane, wherein the first gas comprisesan alkene, an alkyne, an alcohol, an aldehyde, or a combination thereof;wherein the IrO₂-based catalyst has the formula of Ir_(x)M_(y)O_(z) orIr_(x)O_(y)X_(z) wherein M is selected from Ru, Ti, Re, Nb, Ta, Os, Pt,Pd, Cu, Ag, Au, Rh, Cr, Mn, Ni, Fe, Co or a combination thereof, where Xis selected from F, Cl, Br, I, S, Se, Te, or a combination thereof,wherein for Ir_(x)M_(y)O_(z) y is greater than 0, z is between 1 and 2and x+y≤1 and wherein for Ir_(x)O_(y)X_(z) x≤1 and y+z is between 1 and2.
 2. The method of claim 1, further comprising prehydrogenation of theIrO₂-based catalyst prior to exposing the alkane to the IrO₂-basedcatalyst; wherein prehydrogenation comprises adsorbing hydrogen onto asurface of the IrO₂-based catalyst to convert at least a portion ofoxygen to OH.
 2. The method of claim 1, wherein the alkane is a C1 to C5alkane.
 3. The method of claim 1, wherein the first gas comprises a C1to C5 alkene, a C1 to C5 alkyne, a C1 to C5 alcohol, a C1 to C5aldehyde, or a combination thereof.
 4. The method of claim 1, whereinexposing comprises exposing the alkane to the IrO₂-based catalyst at atemperature of about 200 to 400 K.
 5. The method of claim 1, wherein theIrO₂-based catalyst comprises IrO₂ deposited onto a support.
 6. Themethod of claim 5, wherein the support is an oxide support selected fromSiO₂, Al₂O₃, TiO₂, MgO, CaO, CeO₂, zeolites, and a combination thereof.7. The method of claim 5, wherein the support is a non-oxide support. 8.The method of claim 1, wherein the base gas is ethane, and the first gasis ethylene.
 9. The method of claim 1, wherein the base gas is methaneand the first gas comprises ethylene, methanol, formaldehyde, or acombination thereof.